The present invention relates to catalysts for the full oxidation of volatile organic compounds (VOC), particularly hydrocarbons, and to a method for the full oxidation of volatile organic compounds (VOC) by using said catalysts.
The total combustion of VOC to CO2 and H2O becomes necessary in view of the toxicity and environmental impact of most unburnt VOC. The goal is to minimize the release of VOC into the atmosphere and the forming of CO, which is in turn a toxic component.
The catalysts used most for VOC combustion are:
a) catalysts based on noble metals, which are characterized by a high cost but also by excellent performance in terms of VOC conversion, and operate at temperatures from 200 to 450xc2x0 C. according to the reactivity of the compound;
b) catalysts based on mixed oxides, typically chromites of copper or of other metals, or barium hexaaluminate, which are characterized by a lower cost but are active in more drastic conditions (temperatures from 400 to 600xc2x0 C.). This second class of catalysts is also used for catalytic combustors for power generation units. In this case, they operate at temperatures above 900xc2x0 C.
The types of catalyst used for the combustion of VOC are not free from drawbacks; high cost (for those based on noble metals) and poor activity (for the second class, accordingly requiring operation at higher temperatures, in conditions in which morphologic or structural transformations are facilitated).
The aim of the present invention is to eliminate the drawbacks of known types of catalyst for the full oxidation of VOC.
In particular, an object of the present invention is to provide catalysts for catalytic combustion which are characterized by high activity, high resistance to temperature, extreme operating conditions, low cost and easy production even as composites and thin films.
Another object of the present invention is to provide catalysts for VOC oxidation with high selectivity for carbon dioxide CO2 with respect to carbon monoxide CO.
Another object of the present invention is to provide catalysts which lead to full oxidation of the VOC in stoichiometric and non-stoichiometric mixtures of VOC and oxygen (oxidizing or reducing conditions), so that the mixture of gases produced due to VOC oxidation contains no carbon monoxide but contains only carbon dioxide.
Another object of the present invention is to provide a method for the full oxidation of VOC which avoids carbon monoxide removal operations and the known negative consequences of the presence of carbon monoxide in the environment.
Another object is to provide a method for oxidizing VOC to CO2 which utilizes the full potential of the VOC oxidation reaction, with evident energy-related advantages.
Another object is to provide a method for eliminating carbon monoxide from gas mixtures that contain it together with oxygen.
This aim, these objects and others which will become apparent from the detailed description of the invention are achieved by catalysts according to the present invention, which comprise one or more non-stoichiometric crystalline compounds conventionally referenced by formulas which respectively correspond to:
1) A14CU24O41 
2) B4Cu5O10 (BCuO2 is also cited in the literature)
and by a method for oxidizing VOC and CO to CO2 according to the present invention, which uses the same catalysts.
In the first of the above cited formulas, A is Sr or a solid solution of Sr with alkaline-earth metals, alkaline metals, lanthanides; in the second formula, B is Ca or a solid solution of Ca with alkaline-earth metals, alkaline metals, or lanthanides.
Examples of catalysts according to the invention have the approximate formula
Sr14Cu24O41
or
Ca4Cu5O10
Both of these compounds and their derivatives by substitution are widely known in the literature (for Ca4Cu5O10 (mentioned as CaCuO2): Roth et al, J Am Ceram Soc, Vol. 72, p. 1545 (1989); for Sr14Cu24O41: McCarron et al, Mat Res Bull, Vol. 23, p. 1355 (1988)), and particularly for the compound Sr14Cu24O41 there is a vast body of literature associated with its unusual electronic properties. Although it is not possible to formulate exactly the above components, they are unequivocally distinguished by their chemical-physical properties and particularly by the powder diffraction profiles, which correspond to the ones listed in JCPDS international powder diffraction tables, on cards 43-0025 and 46-0054 for the compounds referred to as Sr14Cu24O41 and CaCuO2, respectively.
The fixing properties of said compounds and derivatives thereof have been described in patent application BO98A600593 herein incorporated by reference. The same patent application describes the use of said compounds to fix gases and gas fixing devices which comprise said compounds. The inventors have now found that surprisingly said compounds, if prepared so as to have a large specific surface area, preferably larger than 25 m2/g (as measured by the BET method), act as catalysts for VOC oxidation. The inventors have found that the reaction for full oxidation of VOC in the presence of the catalysts according to the invention occurs with a high conversion of VOC even at low temperatures.
Moreover, the inventors have found that the catalysts according to the present invention allow VOC oxidation (even when the conversion is partial) with total selectivity toward the forming of CO2 even in conditions in which there is a significant deficit of oxygen with respect to the stoichiometric mix. The expression xe2x80x9ctotal selectivityxe2x80x9d is used to reference the fact that VOC oxidation occurs until only CO2 and H2O are obtained. In other words, in conditions of oxygen deficit, while the quantity that corresponds stoichiometrically to the quantity of oxygen that is present is converted into CO2 and H2O, the other fraction of VOC remains unconverted.
The temperature ranges over which the catalytic oxidation process is active depend crucially on the volatile organic compound to be oxidized. Considering methane as the most stable and least easily oxidizable hydrocarbon, the activation temperatures of the reaction for full oxidation of methane constitute the upper limit of the activation temperatures for any VOC: in particular, the activation temperatures of methane are from 300xc2x0 C. to 350xc2x0 C. and from 350xc2x0 C. to 400xc2x0 C. for the compounds A14Cu24O41 and B4CuSO10, respectively. The maximum utilization temperatures of the catalysts instead correspond to the decomposition temperatures of the compounds A14Cu24O41 and B4Cu5O10, which are proximate to 1000xc2x0 C. and 750xc2x0 C., respectively.
The methods for full oxidation of VOC according to the present invention may be performed in combustion chambers or in reheat chambers or flue gas chambers.
The catalytic oxidation reaction of the catalysts according to the present invention occurs on a fixed bed or on a fluid bed.
The catalysts according to the present invention can be in the form of granules.
Advantageously, the catalysts according to the present invention include a substrate material. The substrate can be an inert substrate in the form of a thin film or a composite material. Preferably, the substrate material is constituted by porous substrates which are inert with respect to the above described active materials, such as Al2O3, TiO2, ZrO2, CeO2, MgO, on which the active material is deposited by impregnation with the aqueous solution of soluble salts (for example nitrates or citrates or acetates or mixtures thereof) of the constituent metals in the correct stoichiometric ratios.
The catalysts according to the invention preferably comprise 5 to 20% by weight of a non-stoichiometric crystalline compound, conventionally designated by a formula which corresponds to Al4Cu2441 (I), where A is Sr or a solid solution of Sr with alkaline-earth metals, alkaline metals, lanthanides; or a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to A4Cu5O10 (II), where A is Ca or a solid solution of Ca with alkaline-earth metals, alkaline metals, lanthanides; or mixtures thereof.
A catalyst comprising a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to Sr14Cu24O41 can be prepared, for example, with a method according to the invention which comprises the steps of:
a) immersing a pre-dried granular porous substrate material in an aqueous solution with a molar concentration of Sr(NO3)2 from 0.23 M to 0.93 M and a molar concentration of Cu(NO3)2 from 0.39 M to 1.59 M;
b) drying at a temperature from 80xc2x0 C. to 120xc2x0 C.;
c) holding at a temperature from 650xc2x0 C. to 750xc2x0 C. in a gas stream which contains oxygen until complete decomposition of the nitrates occurs.
A catalyst comprising a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to Ca4Cu5O10 is prepared with a method according to the invention which comprises the steps of:
a) immersing a pre-dried granular porous substrate material in an aqueous solution of Ca(NO3)2 and Cu(NO3)2 in an equimolar ratio and at a molar concentration from 0.39 M to 1.39 M;
b) drying at a temperature from 80xc2x0 C. to 120xc2x0 C.;
c) holding at a temperature from 650xc2x0 C. to 750xc2x0 C. in a gas stream which contains oxygen until complete decomposition of the nitrates occurs.
Furthermore, a catalyst comprising a non-stoichiometric crystalline compound conventionally designated by a formula which corresponds to Ca4Cu5O10 is prepared with a method according to the invention which comprises the steps of:
a) immersing a pre-dried granular porous substrate material in an aqueous solution obtained by dissolving, with the application of heat, CuO and CaCO3 in nitric acid, so that the molar ratio between the components of the solution is CuO: CaCO3:HNO3=1:0.83:3.2; water and citric acid is added thereto so that the citric acid: Cu molar ratio is from 3.5:1 to 4.0:1;
b) heating in air until combustion of the organic fraction of the absorbed material is achieved;
c) thermal treatment for 4 to 24 hours at a temperature from 650 to 750xc2x0 C. in a stream of gas containing oxygen.
Preferably, the porous material that is used is constituted by Al2O3, ZrO2, CeO2, TiO2, MgO.
However, one should not consider the present invention to be limited to catalysts prepared with the described methods.